Method of making cylindrical mesh electrode for electron tubes



J. P. POLESE Filed Feb. 1, 1967 INVENTOR. JAMES P POLESE ATTORNEY Sept.9, 1969 METHOD OF MAKING CYLINDRICAL MESH ELECTRODE FOR ELECTRON TUBESUnited States Patent M 3,465,400 METHOD OF MAKING CYLINDRICAL MESHELECTRODE FOR ELECTRON TUBES James P. Polese, Menlo Park, Calif.,assignor to Varian Associates, Palo Alto, Calif, a corporation ofCalifornia Filed Feb. 1, 1967, Ser. No. 613,221 Int. Cl. Hltlj 9/60 US.Cl. 29-25.14 11 Claims ABSTRACT OF THE DISCLOSURE Background of theinvention This invention relates to an improved method of makingcylindrical cathode and grid electrodes for use in electron tubes andparticularly to an improved method of making wire mesh cathodes andgrids.

Cathode and grid electrodes of conventional high power electron tubescommonly comprises cylindrical arrays of wires. In the prior art, it hasbeen proposed that such arrays of wires be made in the form of acylindrical mesh, and that for structural reasons the individual wiresof the cylindrical mest electrodes be joined together at their crossingpoints. Heretofore this has been done by spotwelding or brazing thewires together. However, such electrodes have often had hundreds ofcrossing points. Methods which separately unite each of such points areobviously cumbersome and impractical, and produce nonuniform structures.Furthermore, the heat of spot-welding, and the alloying resulting frombrazing embrittles the wires or introduces other non-uniformities in themechanical characteristics of the electrode. Accordingly, an object ofthe present invention is the provision of a method of making cylindricalwire mesh electrodes wherein all intersecting wires are joined togetherin one single operation and at a temperature below that which wouldembrittle the wires or introduce other non-uniformities in themechanical characteristics of the electrodes.

Even if great care is exercised in spot-welding the intersecting wiresto avoid the introduction of non-uniform mechanical characteristics, theresulting structure will have certain non-uniformities in mechanicalcharacteristics due to stresses inherently present in metal members as aresult of prior fabrication processes. One of the advantages of themethod of the present invention is a forming action which results fromthe method and which relievesthe stresses present in the wire thusdecreasing if not eliminating the memory of the wire and the attendantwarping or distortion of the electrode when it is heated duringsubsequent operation of the electron tube. This is accomplished by thepresent method without an accompanying embrittlement and weakening ofthe wires at their intersections.

A directly-heated cathode comprising wires which will both emitelectrodes and exhibit high electrical resistance is inherently thefastest heating cathode structure available. However, directly-heatedcathodes constructed in accordance with the prior art are not capable ofas much total electron emission at operating temperature asindirectly-heated cathodes, even if the wires of the directly- 3,465,400Patented Sept. 9, 1969 heated cathode structures are coated with amaterial capable of high electron emission. This is due to the limitedsurface area afforded by the wires to act as a supporting base for theemissive material. Such surface area may be increased by increasing thenumber of wires but the crosssectional area of each wire must then bereduced in order to maintain the required high electrical resistance,and the resulting structure would comprise a greatly increased number ofwires of much smaller cross-sectional dimensions. Although a cylindricalmesh of a large number of extremely fine wires coated with a highlyemissive material would produce an extremely quick-heating cathodehaving total electron emission capability approaching that of anindirectly-heated cathode, such a structure could not be made accordingto the teaching of the prior art for the reasons set forth above. Themethod of the present invention is uniquely suitable for use in making aquick heat cathode for an electron tube.

Summary of the invention Briefly described the present inventioncomprises the steps of forming a cylindrical mesh by winding a wire ofrefractory metal such as tungsten or molybdenum which may be coated witha soft metal such as nickel or gold about a mandrel having a highercoefiicient of thermal expansion than the refractory metal, heating thewire and mandrel to a temperature below the melting point of the wireincluding the coating, but high enough to produce sufiicient pressure tocause the wire-to-wire contacts to fuse, cooling and removing the bondedmesh structure from the mandrel. Where the electrode is used as acathode, nickel is preferred as the wire coating; where the electrode isused as a grid, gold is preferred as the wire coating. Electrodemounting rings, made of a refractory metal such as molybdenum and coatedwith the soft metal with which the refractory wires are coated, may beplaced on the mandrel in spaced relation to each other prior to thewinding. The cylindrical mesh is then formed between the two rings whenthe wire is wound onto the mandrel over the rings. Upon heating allwire-to-wire contacts fuse together simultaneously with the fusing ofthe wireto-wire contacts. Sectional cylindrical clamps may be placedover and held tightly to the wire-wound mandrel. Upon heating the clampsproduce more pressure at the wire-to-wire and wire-to-ring contacts, anddistribute the pressure more evenly over the mesh.

The invention possesses other features which will become apparent fromthe following detailed description and the accompanying drawing. Itshould be understood, however, that the invention is not limited to theembodiment of the method or the means described and illustrated, fornumerous embodiments fall within the scope of the claims.

Brief description of the drawings FIGURE 1 is a perspective view of oneelectrode made in accordance with the method of the present invention.

FIGURE 2 is a perspective view of another electrode made in accordancewith this invention.

FIGURE 3 is an end view in elevation of a modified version of theelectrode shown in FIGURE 1.

FIGURE 4 is a perspective view of a mandrel about which an electrode issupported and shows disassembled clamping means which may be used inpracticing the invention.

FIGURE 5 is a sectional view taken along line 55 in FIGURE 4.

Description of the preferred embodiments Referring now in more detail tothe drawing in which like reference characters designate like orcorresponding parts throughout the several views, there is illustratedin FIGURE 1 an electrode 8 made in accordance with the presentinvention. An upper cylindrical ring 12 is joined to a lower cylindricalring 14 by a wire mesh 16 forming a hollow right circular cylindricalelectrode. Where the electrode is to be employed as a cathode, rings 12and 14 are preferably made of nickel-plated molybdenum and the wire mesh16 of nickel-plated tungsten. Where the electrode is to be used as agrid, gold is used in lieu of nickel and the wire is preferablygold-plated molybdenum. All individual wire-to-wire and wire-to-ringjunctions are fused together resulting in a rigid structure.

FIGURE 2 illustrates an electrode 9 similar to that shown in FIGURE 1except that ring 12 is in the form of a centrally apertured disk. Thisconfiguration results in less end cooling where the electrode is used asan oxide cathode.

FIGURE 3 illustrates an electrode in plan view which is electrode 8 ofFIGURE 1 modified to have a planar wire mesh screen 17 partially closingone end. Such screens are typically used for electron shieldingpurposes. As in electrodes 8 and 9, all wire-to-wire junctions are fusedtogether. If desired, electrode 9 may be similarly modified.

FIGURE 4 depicts the apparatus and parts used to fabricate electrode 8.A cylindrical mandrel is shown having an enlarged portion formingshoulders 22 with unenlarged portions 28 thereof. The mandrel is made ofa material which is stable in form, which will not contaminate or fuseto soft metal-clad wires subsequently wound about it, and which has asuitable higher coefficient of thermal expansion than such wire.Although there are several materials which satisfy these requirements, ahigh alumina ceramic has been found to be quite suitable. Whereelectrodes 9 or 10 are to be fabricated, each of which have one endpartially obstructed either by ring 12' or screen 17, mandrel 20terminates at one of shoulders 22. This modified version of the mandrel,not shown, is further designed to axially receive a screw at thisterminus. The screw is used either to secure a centrallyapertureddisk-shaped clamp to the end of the mandrel for purposes which will behereafter explained, or to hold ring 12' to the mandrel while mesh 16 isbeing formed. In fabricating either electrodes 8, 9 or 10 one unenlargedportion 28 of the mandrel is connected to a wire-winding machinewhenever mesh 16 is not to be formed by hand. In FIGURE 4 rings 12 and14 are shown mounted at each end of enlarged portion 30 of the mandreladjacent shoulders 22. This is their disposition for the fabrication ofeither electrode 8 or 10. Where electrode 9 is to be formed, ring 12 isplaced upon, and held by the aforementioned screw means to shoulder 22.Rings 12, 12' and 14 are preferably made of nickel-clad molybdenum, butmay also be gold-clad molybdenum.

A continuous wire 18 is shown helically wound about enlarged portion 30of mandrel 20 in FIGURE 4 forming mesh 16. Where electrode 8, 9 or 10 isto be used as a cathode, wire 18 is made of tungsten having a diameterof approximately .0005 inch. Tungsten is preferred because of itselectrical resistance, structural strength, and expansion properties.The tungsten wire is clad with approximately .0001 inch of nickel whichprovides a good base for electron emissive materials. The nickelcoatings of intersecting wires will also fuse together without meltingat a temperature below that at which tungsten embrittles. Whereelectrode 8, 9 or 10 is to be used as a grid, gold-coated molybdenum isused in lieu of nickelclad tungsten. Gold is preferred as the soft metalfor its electron suppression properties and molybdenum is preferredbecause of its strength and lower electrical resistance.

Also shown in FIGURE 4 is clamping means 23 comprising at least threesemi-cylindrical sections 24 preferably made of ceramic, and tieing wire25. The radius of sections 24 is made greater than the radius ofenlarged portion 30 of the mandrel by an amount corresponding to thesize of the wire Wound on the mandrel. The sum of the angles subtendedby the arcs of sections 24 should be slightly less than 360. Suchdimentional relationships between the mandrel and clamps will allow forthe semicylindrical sections to fit closely about the surface of themandrel while allowing room between adjoining sections for concentriccompression caused by a subsequent tightening of tieing wire 25. Thetieing wire is made of a refractory material having a lower coeificientof thermal expansion than the mandrel such as molybdenum.

Finally in FIGURE 4 there is also shown indentures 26 along the insideedges of semi-cylindrical sections 24 dimensioned to receive and houserings 12 and 14 when clamping means 23 are afiixed about mandrel 20.Indentures 26 serve the purpose of distributing the pressure caused by atightening of clamping means 23 about the mandrel evenly upon both mesh16 and rings 12 and 14 which lie between the mandrel and clamps.

Referring now to FIGURE 5 there is shown, in plan view, one shoulder 22of mandrel 20 with several sections of continuous wire 18 shown loopedover the shoulder in chordal fashion. This shoulder is used to supportthe wire while mesh 16 of electrode 8 is being wound and before the meshand rings are fused into an integral structure. Shoulder 22 serves thissame purpose during the formation of electrode 9 of FIGURE 2. In thislatter case, however, ring 12 is laid directly upon and held toshoulder'22. Wire 18 is then looped over the ring in a chordal manner.Thus for the fabrication of the embodiment shown in FIGURE 2, referencenumeral 12' should be substituted for reference numeral 22 in FIGURE 5.

Having fully described the embodiments shown in the drawing and theapparatus used in fabricating them, the actual steps of fabrication willnow be explained.

Rings 12, 14 and wire 18 may be formed by electroetching the refractorymetal base to clean and size it, and then electroplating the nickel orgold thereon. Such rings and wires are also available commercially. Forthe fabrication of electrode 8 rings 12 and 14 are placed on enlargedportion 30 of mandrel 20. Wire 18 is then helically wound about enlargedportion 30 of the mandrel and over rings 12 and 14, preferably by a wirewinding machine. Where the wire reaches an end of enlarged portion 30 itis brought across shoulder 22 in a chordal manner before again crossingenlarged portion 30. This continuous winding of Wire over enlargedportion 30 and shoulders 22 is repeated until mesh 16 is completelyformed. During the formation of the mesh shoulders 22 hold wire 18 tautthereby preventing slippage of the wire along the surface of enlargedportion 30 of the mandrel. In this manner, the configuration of mesh 16is maintained while it is being formed and before it is fused into anintgeral structure.

When mesh 16 has been formed the mandrel is removed from the wirewinding machine, if one has been used. Clamping means 23 are then placedover rings 12, 14 and mesh 16, and secured about enlarged portion 30 ofthe mandrel. In this manner all wire-to-wire and wireto-ring contacts ofthe mesh electrode are squeezed together. The clamped mandrel is thenplaced in a furnace having a wet hydrogen atmosphere. The hydrogenatmosphere serves to prevent oxidation during heating while the watervapor serves to keep the structure clean by removing carbon which wouldhamper fusion of the intersecting wires and rings. The clamped mandrelis then heated to approximately 1030 C. for some 20 minutes. Due to thisincrease in temperature all materials in the assembly expand. Themandrel, however, expands more than mesh 16 or rings 12 and 14 due tothe differential in their coefiicients of thermal expansion. The meshand rings, trapped between the expanding mandrel and the clamping meanswhich expand to a lesser degree than the mandrel because of the lowercoefiicient of thermal expansion of its tieing wire, are subjected tosubstantial pressure. This pressure would of course, occur even withoutuse of the clamps for as the mesh is an integral structure consisting ofone continuous wire having a coefficient of thermal expansion less thanthe mandrel, it must resist enlargement under the force of the expandingmandrel about which it is wound. This resistance places all wire-to-wireand wire-to-ring contacts under pressure. As the mandrel expands bothradially and longitudinally in the furnace, both those sections ofcontinuous wire 18 which lie over the enlarged portion of mandrel 30 andover shoulders 22 are also stressed. However, ceramic sections 24, heldagainst the expansion of the mandrel by the highly refractory tieingwire 25, apply still more force against the mesh and rings, and moreimportantly, distributes the pressure evenly. As a result of thiscomhination of time, temperature and pressure, the elastic limit of wire18 is surpassed. All the nickel wire-to-wire and wire-to-ring contactsfuse together. Following this the assembly is cooled and removed fromthe furnace. The clamping means and the excess wire extending beyond therings and lying across shoulders 22 are removed. The structure can thenbe easily slipped off the mandrel since it has been permanentlystretched in the furnace. If the electrode is to be used as a cathode,it is then sprayed with an electron emissive coating such as barium,strontium or calcium carbonate, .or a combination thereof, which, uponsubsequent heating, is converted to an electron emissive oxide. Theelectrode is now ready for assembly into an electron tube.

As the electrode has been stretched beyond its elastic limit in thefurnace, it has lost its memory of prior forms. This results in a sound,symmetrical structure which will retain its shape when later heated inan electron tube, and which has not been embrittled during fabrication.As all intersecting wires have been fused together in a singleoperation, the number of wires comprising the mesh can be greatlyincreased over prior art method in which each individual intersectionwas separately joined. This increase in number enhances the strength ofthe mesh structure, and increases the electron emissive material basearea. Several thousand wire intersections may quite easily 'be fused inthe use of this invention.

Electrode 9 is made in the same manner as described above except thatring 12' is placed on shoulder 22 in lieu of ring 12 being placed onenlarged portion 30 of the mandrel. After wire 18 has been wound ontothe mandrel, a centrally-aperture disk-shaped clamp may be placed overring 12' and secured to the terminal end of enlarged portion 30 of themandrel by screw means. Upon subsequent heating in the furnace the wiresoverlapping ring 12' fuse thereto. Upon cooling electrode 9 is slippedoff enlarged portion 30 which, in the apparatus used to form thisembodiment, terminates at one shoulder 22.

Electrode may also be made using the method by which electrode 8 ismade. For this embodiment however the modified mandrel used infabricating electrode 9 is again employed. The variation in procedureconsists of lengthening the chords formed by wire 18 which form screen17. A comparison between FIGURES 3 and 5 will reveal this increase inlength.

In summary, the above-described steps provide a simple, economicalmethod of producing a rugged, symmetrically cylindrical mesh electrodehaving hundreds of wires fused together at several thousand junctions.The electrode may be used as either a cathode or a grid in low or highpower electron tubes. It is particularly useful as an instant heatcathode for use in tubes which must become operative quickly as, forexample, those used in pushto-talk transmitters.

Those skilled in the art will appreciate the fact that materials otherthan those specifically recommended above may be used in practicing thisinvention, and that numerous combinations of the time, temperature andpressure variables may be employed without deviating from the scope ofthe invention or from the spirit of the following claims.

What is claimed is:

1. The method of making a cylindrical electrode of refractory wire meshhaving fused intersecting wires for use in an electron tube comprisingthe following steps:

(A) forming a hollow, cylindrical mesh structure by winding a refractorywire about a mandrel, said wire having a coefficient of thermalexpansion less than that of said mandrel,

(B) heating the mesh and mandrel to a temperature in the range abovethat at which sufficient pressure is exerted by the expanding mandrelagainst the mesh structure to cause the wire-to-wire contacts of themesh to fuse, and below that at which the wire melts,

(C) cooling and removing the fused mesh structure from the mandrel.

2. The method according to claim 1 wherein said refractory wire iscoated with a soft metal prior to winding on the mandrel and saidheating is to a temperature below the melting point of said soft metal.

3. The method of claim 2 wherein said electrode is a cathode and whereinsaid coating step comprises the coating of nickel on a tungsten wire.

4. The method of claim 3 wherein said tungsten wire is drawn toapproximately .0005 inch in diameter and coated with nickel to a depthof approximately .0001 inch.

5. The method of claim 3 wherein said electrode is coated with anelectron emissive material after removal from the mandrel.

6. The method of claim 5 wherein said electrode is coated with anemissive material selected from'the group consisting of bariumcarbonate, strontium carbonate, calcium carbonate, and combinationsthereof.

7. The method of claim 3 wherein two spaciallyseparated nickel-coatedmolybdenum rings are placed on said mandrel prior to step A, and whereinthe heating of step B causes the wire-to-ring contacts to fuse.

8. The method of claim 2 wherein said electrode is a grid and whereinsaid coating step comprises the coating of gold on a molybdenum wire.

9. The method of claim 1 wherein said wire is wound over one end of saidmandrel.

10. The method of claim 1 wherein following step A clamping means areplaced about and held tightly to said mesh against the thermal expansionof said mandrel.

11. The method of claim 4 wherein said heating is conducted in a wethydrogen furnace at a temperature of approximately 1030 C. forapproximately 20 minutes.

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